insert background image in HIFI template

Hello i have a question i try to insert background image in jp HIFI template.
is it possible to give me the name of file may be in css folder but i don't know which file plus is possible to give code to insert thank you

Please never edit the original CSS files, because these files will be overwritten with the next theme update. Always add your custom style to the custom.css
www.joomlaplates.com/template-customizing

Open the css/custom.css and insert your code for full size background.
www.w3schools.com/css/css_background.asp

We are committed to help you with any type of support query, however we can't provide extensive help in terms of customizations. In general customizations may look simple but you will require solid knowledge of web technologies and programming skills to do them or hire a professional who can do it for you.